Nonaqueous electrolyte secondary battery

ABSTRACT

The present invention aims at improving both of power characteristics and a capacity retention ratio of a nonaqueous electrolyte secondary battery. A nonaqueous electrolyte secondary battery according to the present invention is characterized in that a negative electrode contains an aqueous binder as a binder and amorphous carbon as a negative active material, and an average particle size of the amorphous carbon is set to a specific average particle size, which is 7 μm or less. By employing a constitution of this characteristic, both of power characteristics and a capacity retention ratio of the nonaqueous electrolyte secondary battery can be improved.

TECHNICAL FIELD

The present invention relates to a nonaqueous electrolyte secondarybattery.

BACKGROUND ART

In recent years, high performance batteries are positively beingdeveloped in association with downsizing and improvement in performanceof electronic equipment such as cellular phones and mobile audioequipment, and a demand of a secondary battery capable of repeatedlyusing by charging is significantly increased. Particularly, a nonaqueouselectrolyte secondary battery exhibiting high energy density and a highoperating voltage, such as a lithium ion secondary battery, receivesattention, and is widely used.

In such a nonaqueous electrolyte secondary battery, each electrodeincludes an active material supported on a current collector made of aconducting material as a main constituent. The positive electrodeincludes a positive active material supported on a positive currentcollector, and the negative electrode includes a negative activematerial supported on a negative current collector. In each electrode, abinder is used for binding the positive active materials or the negativeactive materials together.

Meanwhile, when high input/output characteristics are required in thenonaqueous electrolyte secondary battery, as described in PatentDocument 1, amorphous carbon may be used as a part of the negativeactive material (refer to paragraph [0016] or the like). In this case,conventionally, solvent type binders typified by fluorine-based polymerssuch as polyvinylidene fluoride (PVdF) have been chiefly used as thebinder for binding the amorphous carbons as a negative active material(refer to paragraph [0049] or the like).

PRIOR ART DOCUMENT Patent Document

-   Patent Document 1: JP-A-2009-193924

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

In general, when an average particle size of an active material isreduced, power characteristics of a battery tend to increase. On theother hand, when the average particle size of an active material isreduced, an area of a reaction of the active material with thenonaqueous electrolyte increases in association with an increase in aspecific surface area of the active material. This causes moredecomposition reactions of a nonaqueous electrolyte, and thus there is apossibility of causing a problem that the capacity retention ratio ofthe battery is deteriorated. For example, the power characteristics arealso improved by reducing the average particle size of the amorphouscarbon when the amorphous carbon is used for the negative activematerial, but the capacity retention ratio can be deteriorated.Therefore, when the amorphous carbon is used for the negative activematerial, the average particle size of the amorphous carbon has beenforced to be set to some large value in order to ensure the capacityretention ratio bearing a practical use. As a result of this, thebattery is in a situation where setting of a particle size for ensuringa predetermined capacity retention ratio becomes a bottleneck and asignificant improvement of power characteristics cannot be expected.

Thus, it is desired to improve both of power characteristics and acapacity retention ratio in the nonaqueous electrolyte secondary batteryusing amorphous carbon as the negative active material.

Means for Solving the Problems

A constitution and the operation and effect of the present inventionwill be described including technological thought. However, an operatingmechanism includes presumption, and its right and wrong does not limitthe present invention. In addition, the present invention may beembodied in other various forms without departing from the spirit andmain features. Therefore, embodiments and examples described later aremerely exemplifications in all respects and are not to be construed tolimit the scope of the invention. Moreover, variations and modificationsbelonging to an equivalent scope of the claims are all within the scopeof the invention.

A first aspect of the present invention is a nonaqueous electrolytesecondary battery including a negative electrode including amorphouscarbon as a negative active material and a binder, wherein the bindercontains an aqueous binder, and wherein an average particle size of theamorphous carbon is 7 μm or less.

According to such a constitution, a nonaqueous electrolyte secondarybattery having excellent power characteristics and an excellent capacityretention ratio can be provided.

That is, as described later, the present inventors made earnestinvestigations, and consequently found that in the battery including thenegative electrode including the amorphous carbon as a negative activematerial, when an aqueous binder is used as a binder contained in thenegative electrode, a surprising event occurs. The surprising event isunpredictable from conventional technical common knowledge. The event inwhich with a reduction of the average particle size of the amorphouscarbon, power characteristics are improved, and the capacity retentionratio turns from decrease to increase at a specific average particlesize as a boundary. It is distinct from the case of using a solvent typebinder. Further, the present inventors found that the specific averageparticle size as the boundary exists within a range of about 10 to 20μm.

That is, the nonaqueous electrolyte secondary battery according to thepresent invention is characterized by combining the negative electrodeincluding an aqueous binder as a binder with the amorphous carbon havingan average particle size of 7 μm or less which is smaller than the abovespecific average particle size as a negative active material, and byemploying a constitution of this feature, power characteristics areimproved, and a capacity retention ratio is improved contrary toconventional technical common knowledge. Particularly, the capacityretention ratio can be significantly improved comparing with the case inwhich the amorphous carbon as a negative active material is used incombination with the solvent type binder.

Advantages of the Invention

According to the present invention, it is possible to provide anonaqueous electrolyte secondary battery having excellent powercharacteristics and an excellent capacity retention ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of an aspect of a nonaqueouselectrolyte secondary battery of the present invention.

FIG. 2 is a schematic view showing an energy storage apparatus includingthe nonaqueous electrolyte secondary battery of the present invention.

FIG. 3 is a schematic view showing an automobile equipped with theenergy storage apparatus including the nonaqueous electrolyte secondarybattery of the present invention.

MODE FOR CARRYING OUT THE INVENTION

In a second aspect of the present invention, the aqueous binder containsat least one selected from among rubber-like polymers and resin-basedpolymers which can be dissolved or dispersed in a water-based solvent inthe nonaqueous electrolyte secondary battery according to the firstaspect. When such a constitution is employed, it is preferred sincepower characteristics and a capacity retention ratio are more improved.

In a third aspect of the present invention, an interlayer distance d₀₀₂,which is determined by a wide angle X-ray diffraction method, of theamorphous carbon is 3.60 Å or more in the nonaqueous electrolytesecondary battery according to the first or second aspect. When such aconstitution is employed, it is preferred since power characteristicsare more improved.

In a fourth aspect of the present invention, the negative electrodeincludes a thickner, and wherein the thickner contains a cellulose-basedpolymer in the nonaqueous electrolyte secondary battery according to anyone of the first to the third aspects.

In a fifth aspect of the present invention, the cellulose-based polymerincludes a carboxymethyl cellulose in the nonaqueous electrolytesecondary battery according to the fourth aspect.

In a sixth aspect of the present invention, a degree of etherificationof the cellulose-based polymer is 1 or less in the nonaqueouselectrolyte secondary battery according to the fourth or fifth aspect.

A seventh aspect of the present invention is an assembled batteryincluding a plurality of the nonaqueous electrolyte secondary batteriesaccording to any one of the first to the sixth aspects.

An eighth aspect of the present invention is an energy storage apparatusincluding the assembled battery according to the seventh aspect.

A ninth aspect of the present invention is an automobile equipped withthe energy storage apparatus according to the eighth aspect.

Embodiments of a nonaqueous electrolyte secondary battery according tothe present invention will be described in reference to drawings. In thepresent embodiment, an example of applying the present invention to alithium ion secondary battery in which lithium ions contained in thenonaqueous electrolyte play a role of electric conduction, will bedescribed. Further, in the present embodiment, an example in which thepresent invention is applied to a prismatic lithium ion secondarybattery will be described. In addition, in the following description, anexplanation about an operating mechanism includes presumption, and itsright and wrong does not limit the present invention.

As shown in FIG. 1, a nonaqueous electrolyte secondary battery 1includes a power generating element 2, a nonaqueous electrolyte (notshown) and a battery case 6 housing these. The power generating element2 is an element functions as the core for discharge and charge, and isformed by including a positive electrode 3, a negative electrode 4 and aseparator 5. In the present embodiment, the power generating element 2is configured by winding the positive electrode 3 and the negativeelectrode 4 with the separator 5 interposed.

The negative electrode 4 includes a negative current collector and anegative composite layer formed on the negative current collector. Thenegative composite layer can contain a negative active material and abinder. The negative composite layer may contain a conduction aid asrequired. The negative composite layer can be formed by applying anegative composite (negative electrode paste) prepared by mixing thesematerials using, for example, a proper solvent appropriate to propertiesof the binder to the negative current collector, and drying thecomposite. In so doing, a thickness or a porosity of the layer can beadjusted by roll pressing.

The negative current collector is configured by using a conductingmaterial. The negative current collector can be formed by using a metalmaterial such as copper, nickel, stainless steel, or Ni electroplatedsteel. Further, as a shape of the negative current collector, variousshapes such as a sheet (foil or thin film), a plate, a columnar body, acoil, a foam, a porous body and an expanded grid, can be employed.

The negative active material is not particularly limited with limits ofcapability of reversibly absorbing/releasing lithium ions. Examples ofthe negative active material include metallic lithium; lithium titanatesuch as Li₄Ti₅O₁₂; graphite; and amorphous carbons such as soft carbon(easily graphitizable carbon) and hard carbon (non-graphitizablecarbon). In the present invention, the negative active material includesamorphous carbon in order to realize the nonaqueous electrolytesecondary battery 1 having high input/output characteristics.

Each carbon material can be identified by a value of an interlayerdistance d₀₀₂ determined by a wide angle X-ray diffraction method. Theamorphous carbon in the present invention is a carbon material whoseinterlayer distance d₀₀₂ is 3.40 Å or more. The interlayer distance d₀₀₂is preferably 3.40 Å or more and 3.90 Å or less.

Further, in the amorphous carbon as the negative active material, acarbon net plane becomes smaller and a lamination of the plane becomesdisordered as the interlayer distance d₀₀₂ increases exceeding 3.40 Å.

Thereby, insertion/extraction of lithium ions between layers becomeseasy, leading to an improvement of power characteristics of a battery.Therefore, the interlayer distance d₀₀₂ of the amorphous carbon as thenegative active material is more preferably 3.60 Å or more and 3.90 Å orless.

The amorphous carbon as the negative active material of the presentinvention has an average particle size of 7 μm or less. When the averageparticle size of the amorphous carbon is more than 7 μm and excessivelylarge, there is a possibility that the difficulty in ensuring sufficientpower characteristics may arise in practical use. Therefore, it ispossible to adequately secure practicality by setting the averageparticle size of the amorphous carbon to 7 μm or less.

In addition, when the average particle size of the amorphous carbon isless than 2 μm and excessively small, there is a possibility thatavailability of a material is deteriorated and cost increases.

The average particle size of the amorphous carbon is not particularlylimited with a limit of 7 μm or less; however, the average particle sizeis preferably 6 μm or less, more preferably 5 μm or less, furthermorepreferably 4.5 μm or less, and moreover preferably 4 μm or less.Further, the average particle size of the amorphous carbon is preferably0.5 μm or more, more preferably 1 μm or more, furthermore preferably 1.5μm or more, and moreover preferably 2 μm or more.

The average particle size of the amorphous carbon is a particle size atwhich a cumulative percentage in a particle size distribution on avolumetric basis is 50% (D50). Specifically, a laser diffractionparticle size distribution analyzer (SALD 2200, manufactured by ShimadzuCorporation) is used as a measurement apparatus, and Wing SALD 2200 isused as a measurement control software program. As a specificmeasurement technique, a measurement mode of scattering type isemployed, and a wet cell, through which a dispersion with a measurementobject sample (amorphous carbon) dispersed in a dispersive solvent iscirculated, is irradiated with laser light to obtain a distribution ofscattered light from the measurement sample. Then, the distribution ofscattered light is approximated by a logarithmic normal distribution,and a particle size which corresponds to a degree of cumulative volumeof 50% (D50) is taken as an average particle size. Further, it wasverified that a particle size at which the cumulative percentage in theparticle size distribution on a volumetric basis is 50% (D50) almostagrees with a particle size which is obtained by extracting 100amorphous carbon particles from a SEM image of a plate. In measuring theextracted amorphous carbon particles, extremely large amorphous carbonparticles and extremely small amorphous carbon particles should beavoided.

The conduction aid is a material to be added for the purpose ofimproving electrical conductivity of the negative composite layer, asrequired. As such a conduction aid, various conducting materials can beused. Examples of the conducting materials include carbon materials suchas acetylene black, carbon black and graphite; conductive fibers such asmetal fibers; metal (copper, nickel, aluminum, and silver) powders;conductive whiskers of zinc oxide or potassium titanate; and conductivemetal oxides such as titanium oxide.

The binder (negative electrode binder) is a material to be contained forthe purpose of binding the negative active materials together. Further,the binder also plays a role of binding the negative active material tothe negative current collector. When the conduction aid is contained inthe negative composite layer, the binder plays a role of binding thenegative active material, the negative current collector and theconduction aid together. As such binders, in general, a solvent typebinder for which an organic solvent is used when being mixed with anactive material to form a paste, and an aqueous binder for which awater-based solvent (typically, water) can be used as a solvent, arepresent. In the present invention, the aqueous binder is used as thebinder contained in the negative composite layer.

Further, when the solvent type binder is used as the binder, the solventtype binder is commonly dissolved in an organic solvent such asN-methyl-2-pyrrolidone for use in preparing a paste (composite) as theactive material. Therefore, for example, in order to lessen the burdenon the environment, it becomes necessary to recover the organic solventas far as possible to reduce an amount of emission of the organicsolvent. As a result of this, it takes much cost such as initial costfor equipment investment and operational cost for operating/controllingequipment.

When the aqueous binder is used as the binder contained in the negativecomposite layer like the present invention, it is unnecessary to recoverthe water-based solvent for forming a paste of the negative composite,and therefore it becomes possible to lessen the burden on theenvironment at low cost.

The aqueous binder is defined as a binder capable of using a water-basedsolvent in preparing a composite (electrode paste). More specifically,the aqueous binder is defined as a binder for which water or a mixedsolvent predominantly composed of water can be used as a solvent inbeing mixed with an active material to form a paste. As such a binder,non-solvent type various polymers can be used.

As the aqueous binder contained in the negative composite layer, atleast one selected from among rubber-like polymers and resin-basedpolymers which can be dissolved or dispersed in the water-based solvent,is preferably used. Herein, the water-based solvent refers to water or amixed solvent predominantly composed of water. As a solvent, other thanwater, constituting the mixed solvent, organic solvents which can beuniformly mixed with water (lower alcohols, lower ketones, etc.), can beexemplified.

Examples of the rubber-like polymers which can be dissolved or dispersedin the water-based solvent include a styrene-butadiene rubber (SBR), anacrylonitrile-butadiene rubber (NBR), a methylmethacrylate-butadienerubber (MBR), and the like. These polymers can be preferably used in astate of being dispersed in water as a binder. That is, examples of theaqueous binder which can be used include a water-dispersed matter of thestyrene-butadiene rubber (SBR), a water-dispersed matter of theacrylonitrile-butadiene rubber (NBR), and a water-dispersed matter ofthe methylmethacrylate-butadiene rubber (MBR). Further, among theserubber-like polymers which can be dissolved or dispersed in thewater-based solvent, the styrene-butadiene rubber (SBR) is preferablyused.

Examples of the resin-based polymers which can be dissolved or dispersedin the water-based solvent include acrylic resins, olefinic resins, andfluorine-based resins. Examples of the acrylic resins include acrylicacid esters, methacrylic acid esters and the like. Examples of theolefinic resins include polypropylene (PP), polyethylene (PE) and thelike. Examples of the fluorine-based resins includepolytetrafluoroethylene (PTFE) and the like. These resins can bepreferably used in a state of being dispersed in water as a binder. Thatis, examples of the aqueous binder which can be used include awater-dispersed matter of the acrylic acid ester, a water-dispersedmatter of the methacrylic acid ester, a water-dispersed matter of thepolypropylene (PP), a water-dispersed matter of the polyethylene (PE),and a water-dispersed matter of the polytetrafluoroethylene (PTFE).

As the aqueous binder contained in the negative composite layer, acopolymer containing, as monomers, two or more of the componentsdescribed above, can also be used. Examples of such a copolymer includean ethylene-propylene copolymer, an ethylene-methacrylic acid copolymer,an ethylene-acrylic acid copolymer, a propylene-butene copolymer, anacrylonitrile-styrene copolymer, a methylmethacrylate-butadiene-styrenecopolymer and the like. These copolymers can be preferably used in astate of being dispersed in water as a binder. That is, examples of theaqueous binder which can be used include a water-dispersed matter of theethylene-propylene copolymer, a water-dispersed matter of theethylene-methacrylic acid copolymer, a water-dispersed matter of theethylene-acrylic acid copolymer, a water-dispersed matter of thepropylene-butene copolymer, a water-dispersed matter of theacrylonitrile-styrene copolymer, a water-dispersed matter of themethylmethacrylate-butadiene-styrene copolymer and the like.

A glass-transition temperature (T_(g)) of the aqueous binder containedin the negative composite layer, is not particularly limited; however,it is preferred when the glass-transition temperature (T_(g)) is −30° C.or higher and 50° C. or lower since the problem-free adhesion andflexibility can be achieved simultaneously during producing a plate andprocessing a plate.

Further, the negative composite layer can include a thickner. Examplesof the thickner include starch-based polymers, alginic acid-basedpolymers, microorganism-based polymers and cellulose-based polymers.

The cellulose-based polymers can be classified into nonionic polymers,cationic polymers and anionic polymers. Examples of the nonioniccellulose-based polymers include alkyl cellulose, hydroxyalkyl celluloseand the like. Examples of the cationic cellulose-based polymers includechlorinated o-[2-hydroxy-3-(trimethylammonio)propyl]hydroxyethylcellulose (polyquaternium-10) and the like. Examples of the anioniccellulose-based polymers include alkyl celluloses having a structurerepresented by the following general formula (1) or general formula (2)formed by substituting the nonionic cellulose-based polymers withvarious derivative groups, and metallic salts or ammonium salts thereof.

In the above general formula (1) and general formula (2), n is a naturalnumber. In the above general formula (2), X is preferably an alkalimetal, NH4 or H. R is preferably a divalent hydrocarbon group. Thenumber of carbon atoms of the hydrocarbon group is not particularlylimited; however, it is usually about 1 to 5. Further, R may be ahydrocarbon group or an alkylene group which contains a carboxy group.

Specific examples of the anionic cellulose-based polymers includecarboxymethyl cellulose (CMC), methyl cellulose (MC), hydroxypropylmethyl cellulose (HPMC), sodium cellulose sulfate, methyl ethylcellulose, ethyl cellulose and salts thereof. Among these celluloses,carboxymethyl cellulose (CMC), methyl cellulose (MC), and hydroxypropylmethyl cellulose (HPMC) are preferred, and carboxymethyl cellulose (CMC)is more preferred.

A degree of substitution of a substitute such as a carboxymethyl groupfor hydroxy groups (three groups) per anhydroglucose unit in thecellulose, is referred to as a degree of etherification, and the degreeof etherification can theoretically assume a value of 0 to 3. A smalleretherification degree shows that the hydroxy group in the celluloseincreases and the substitute decreases. In the present invention, adegree of etherification of cellulose as the thickner contained in thenegative composite layer is not particularly limited; however, thedegree of etherification is preferably 1.5 or less, more preferably 1 orless, furthermore preferably 0.8 or less, and moreover preferably 0.6 orless.

In addition, the negative composite layer may contain other componentssuch as a dispersing agent like a surfactant in addition to theamorphous carbon as the negative active material and the aqueous binderas the binder.

The content of the amorphous carbon in the negative composite layer ispreferably 50% by mass or more with respect to a mass of the negativecomposite layer from the viewpoint of more improving a battery capacity.Further, the content of the amorphous carbon is more preferably 60% bymass or more, furthermore preferably 70% by mass or more, furthermorepreferably 80% by mass or more, and moreover preferably 90% by mass ormore with respect to a mass of the negative composite layer.

A porosity of the negative composite layer is not particularly limited,and it is preferably 50% or less, more preferably 45% or less,furthermore preferably 40% or less, and moreover preferably 35% or less.Further, the porosity of the negative composite layer is preferably 10%or more, more preferably 15% or more, furthermore preferably 20% ormore, and moreover preferably 25% or more.

The positive electrode 3 includes a positive current collector and apositive composite layer formed on the positive current collector. Thepositive composite layer can contain a positive active material, aconduction aid, and a binder. The positive composite layer can be formedby applying a positive composite (positive electrode paste) prepared bymixing these materials using, for example, a proper solvent appropriateto properties of the binder to the positive current collector, anddrying the composite. In so doing, a thickness or a porosity of thelayer can be adjusted by roll pressing.

The positive current collector is configured by using a conductingmaterial. The positive current collector can be formed by using a metalmaterial such as aluminum, copper, nickel, stainless steel, titanium, ortantalum. Further, as a shape of the negative current collector, variousshapes such as a sheet (foil or thin film), a plate, a columnar body, acoil, a foam, a porous body and an expanded grid, can be employed.

The positive active material is not limited with limits of capability ofreversibly absorbing/releasing lithium ions. As such a positive activematerial, for example, a lithium transition metal composite oxidecapable of absorbing/releasing lithium ions can be used. Examples of thelithium transition metal composite oxide include lithium-cobaltcomposite oxide such as LiCoO₂; lithium-nickel composite oxide such asLNiO₂; and lithium-manganese composite oxide such as LiMnO₂, LiMn₂O₄ andLi₂MnO₃. Further, a part of these transition metal atoms may be replacedwith another transition metal or light metal. Or, olivine compoundscapable of absorbing/releasing lithium ions may be used as the positiveactive material. Examples of the olivine compounds include olivine typelithium phosphate compounds such as LiFePO₄.

The conduction aid is a material to be added for the purpose ofimproving electrical conductivity of the positive composite, asrequired. As such a conduction aid, various conducting materials can beused, and the same materials as in the conduction aid described abovecan be employed.

The binder (positive electrode binder) is a material to be added for thepurpose of binding the positive active materials together. Further, thebinder also plays a role of binding the positive active material, theconduction aid and the positive current collector together. As thebinder contained in the positive composite layer, the aqueous binder canbe used, or the solvent type binder can also be used. As the aqueousbinder, the same materials as in the above aqueous binder contained inthe negative composite layer, can be employed.

The solvent type binder is a binder for which an organic solvent is usedwhen being mixed with an active material or the like to form a paste. Asthe solvent type binder, polyvinylidene fluoride (PVdF), polymethylmethacrylate (PMMA), polyacrylonitrile (PAN) or the like can be used.When the solvent type binders are used, they can be preferably used in astate of being dissolved in an aprotic polar solvent which is oneexample of the organic solvent. As the aprotic polar solvent, aproticamide-based solvents such as N-methyl-2-pyrrolidone (NMP) andN,N-dimethylformamide (DMF) can be used.

In addition, the positive composite layer may contain other componentssuch as a thickner and a dispersant as with the negative compositelayer.

The separator 5 separates the positive electrode 3 from the negativeelectrode 4 and retains a nonaqueous electrolyte, and is disposedbetween the positive electrode 3 and the negative electrode 4. As amaterial of the separator, various materials can be appropriately used,and for example, synthetic resin microporous membranes, cloths, nonwovenfabrics or the like can be used. As the synthetic resin microporousmembranes, for example, a microporous membrane made of polyethylene, amicroporous membrane made of polypropylene, or a combined microporousmembrane thereof can be used.

In the nonaqueous electrolyte secondary battery of the presentinvention, an insulating layer may be disposed besides the separatorbetween the positive electrode and the negative electrode. When theinsulating layer is disposed besides the separator between the positiveelectrode and the negative electrode, it is possible to prevent thepositive electrode and the negative electrode from electricallyconnecting to each other even though a usage pattern of the nonaqueouselectrolyte secondary battery is out of a scope of the usage patternusually foreseen. This is because, even when a thermal shrinkage of aseparator causes owing to an abnormal heat which derives from being outof the scope of the usage pattern usually foreseen, the insulating layerremains.

The insulating layer can be an insulating porous layer, and for example,a porous layer containing an inorganic oxide, a porous layer containingresin beads, a porous layer containing a heat-resistant resin such as anaramid resin can be employed. In the nonaqueous electrolyte secondarybattery of the present invention, the insulating layer is preferably theporous layer containing an inorganic oxide. The porous layer containingan inorganic oxide as the insulating layer may contain the binder andthe thickner as required.

The binder and the thickner contained in the porous layer are notparticularly limited, and for example, the same materials as those usedin the composite layer (positive composite layer or negative compositelayer) can be used.

As an inorganic oxide, publicly known ones can be used; however, theinorganic oxides having excellent chemical stability are preferred.Examples of such inorganic oxides include alumina, titania, zirconia,magnesia, silica, boehmite and the like. As the inorganic oxide, apowdery one is preferably used. The average particle size of theinorganic oxide is not particularly limited, and it is preferably 10 μmor less, more preferably 8 μm or less, furthermore preferably 5 μm orless, and moreover preferably 3 μm or less. Further, the averageparticle size of the inorganic oxide is not particularly limited, and itis preferably 0.01 μm or more, more preferably 0.05 μm or more, andmoreover preferably 0.1 μm or more. The inorganic oxide may be usedsingly or may be used in combination of two or more types.

The insulating layer can be formed on one or more of one surface of theseparator, both surfaces of the separator, a surface of the positivecomposite layer and a surface of the negative composite layer. Further,when the insulating layer is formed on the surface of the compositelayer, at least a part of the composite layer may be covered with theinsulating layer, or the whole area of the composite layer may becovered with the insulating layer.

As a method of forming the insulating layer, a publicly known method canbe employed. For example, a method in which a composite for forming aninsulating layer containing an inorganic oxide and a binder is appliedto one or more of one surface of the separator, both surfaces of theseparator, a surface of the positive composite layer and a surface ofthe negative composite layer, and dried, can be employed.

When the inorganic oxide and the binder are contained in the compositefor forming an insulating layer, the content of the binder is notparticularly limited; however, the content is preferably 20% by mass orless, and more preferably 10% by mass or less with respect to a mass ofthe insulating layer. Further, the content of the binder is preferably1% by mass or more, and more preferably 2% by mass or more with respectto a total amount of the inorganic oxide and the binder. By satisfyingsuch a content range, a good balance between mechanical strength andlithium ion conductivity of the insulating layer can be achieved.

A thickness of the insulating layer is not particularly limited, and itis preferably 20 μm or less, and more preferably 15 μm or less. Thethickness of the insulating layer is preferably 2 μm or more, and morepreferably 4 μm or more.

The form in which the insulating layer is formed on the surface (onesurface or both surfaces) of the separator is more preferred than theform in which the insulating layer is formed on the surface of thecomposite layer (positive composite layer or negative composite layer).This is because, in the former, it does not happen that a layer in whichthe composite layer and the insulating layer are mixed with each otheris formed at an interface between the composite layer and the insulatinglayer, and therefore a conductive path in the composite layer is keptgood.

The form in which the insulating layer is formed on a surface facing thepositive electrode of the surfaces of the separator is more preferredthan the form in which the insulating layer is formed on a surfacefacing the negative electrode of the surfaces of the separator since itis possible to prevent the separator from being formed into polyene.

A power generating element 2 formed by including a positive electrode 3,a negative electrode 4, and a separator 5 is housed in a battery case 6.Further, the battery case 6 accommodates a nonaqueous electrolyte, andthe power generating element 2 is impregnated with the nonaqueouselectrolyte.

The nonaqueous electrolyte is obtained by dissolving a supporting saltin a nonaqueous solvent (solvent other than water). As the nonaqueoussolvent, an organic solvent can be preferably used. As such an organicsolvent, for example, carbonates such as dimethyl carbonate (DMC),ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate(PC), butylene carbonate (BC) and ethyl methyl carbonate (EMC); esterssuch as γ-butyrolactone and methyl formate; and ethers such as1,2-dimethoxyethane and tetrahydrofuran, can be suitably employed. Amixed solvent of two or more thereof may be employed.

As the nonaqueous solvent, a molten salt (ionic liquid) may be used. Assuch a molten salt, for example, imidazolium salts such asethylmethylimidazoliumtetrafluoro borate (EMI-BF₄) andethylmethylimidazoliumtrifluoromethane sulfonylimide (EMI-TESI);pyridinium salts such as 1-ethylpyridinium tetrafluoroborate and1-ethylpyridinium trifluoromethanesulfonylimide; ammonium salts such astrimethyl propyl ammonium bis(trifluoromethanesulfonyl)imide(TMPA-TFSI); and sulfonium salts such as triethylsulfoniumbis(trifluoromethanesulfonyl)imide (TES-TFSI), can be used.

As the supporting salt, a lithium salt can be used. As the lithium salt,any of inorganic lithium salts and organic lithium salts may be used.Examples of the inorganic lithium salts include lithium fluoride saltssuch as LiPF₆, LiAsF₆, LiBF₄ and LiSbF₆; lithium chloride salts such asLiAlCl₄; and lithium perhalogenates such as LiClO₄, LiBrO₄ and LiIO₄.Examples of the organic lithium salts include fluorine-containingorganolithium salts. Examples of the fluorine-containing organolithiumsalts include perfluoroalkane sulfonic acid salts such as LiCF₃SO₃ andLiC₄F₉SO₃; perfluoroalkane carboxylic acid salts such as LiCF₃CO₂;perfluoroalkane carbonimido salts such as LiN(CF₃CO)₂; andperfluoroalkane sulfonimido salts such as LiN(CF₃SO₂)₂ andLiN(C₂F₅SO₂)₂. These may be used in combination of two or more thereof.

In addition, vinylene carbonate (VC) or the like may be added to thenonaqueous electrolyte as an additive.

The battery case 6 is configured using a metal material, for example,aluminum or an aluminum alloy. A battery lid 7 is fixed to an opening ofthe battery case 6 and seals the battery case 6 in a state in which thepower generating element 2 and the nonaqueous electrolyte are housed inthe battery case 6.

In the present embodiment, the battery lid 7 doubles as a positiveelectrode terminal. Further, a negative electrode terminal 9 is providedat a central part of the battery lid 7. The negative electrode 4 isconnected to the negative electrode terminal 9 with a negative electrodelead 11 interposed. The positive electrode 3 is connected to the batterylid 7 as the positive electrode terminal with a positive electrode lead10 interposed. In addition, a safety valve 8 for releasing a gasexternally when an internal pressure in a sealed container reaches apredetermined pressure is disposed at the battery lid 7.

In the nonaqueous electrolyte secondary battery 1 as described above,the present invention is characterized by combining the negativeelectrode including an aqueous binder as a binder with the amorphouscarbon having an average particle size of 7 μm or less as a negativeactive material.

Thereby, both of power characteristics and a capacity retention ratiocan be improved. This point will be described in more detail below byway of examples and comparative examples. However, the present inventionis not limited to these examples.

EXAMPLES Example 1

The nonaqueous electrolyte secondary battery 1 of the embodiment shownin FIG. 1 was prepared by the following procedure.

<1> Preparation of Negative Electrode

As a negative active material, amorphous carbon in which an averageparticle size was 5.5 μm and an interlayer distance d₀₀₂, determined bya wide angle X-ray diffraction method, was 3.45 Å, was prepared. Theamorphous carbon (95.3 parts by mass), 2.8 parts by mass of astyrene-butadiene rubber (SBR) as a binder, 1.9 parts by mass ofcarboxymethyl cellulose (CMC) as a thickner, and water were mixed toprepare a negative composite (negative electrode paste). Next, theprepared negative composite was applied to both surfaces of a negativecurrent collector made of a copper foil having a thickness of 10 μm by adoctor blade method to form a negative composite layer on the negativecurrent collector. Thereafter, the negative composite layer was dried toobtain a negative electrode. A negative electrode lead was attached tothe negative electrode.

<2> Preparation of Positive Electrode

A LiFePO₄ powder (88 parts by mass) as a positive active material, 6parts by mass of acetylene black as a conduction aid, 6 parts by mass ofpolyvinylidene fluoride (PVdF) as a binder, and N-methyl-2-pyrrolidone(NMP) were mixed to prepare a positive composite (positive electrodepaste). Next, the prepared positive composite was applied to bothsurfaces of a positive current collector made of an aluminum foil havinga thickness of 20 μm by a doctor blade method to form a positivecomposite layer on the positive current collector. Thereafter, thepositive composite layer was dried to obtain a positive electrode. Apositive electrode lead was attached to the positive electrode.

<3> Preparation of Nonaqueous Electrolyte Secondary Battery

A polyethylene microporous membrane was used as a separator. Anonaqueous electrolyte solution as a nonaqueous electrolyte was preparedby dissolving LiPF₆ as a supporting salt, so as to have theconcentration of 1 mol/l, in a mixed solvent in which a volume ratio ofethylene carbonate (EC)/dimethyl carbonate (DMC)/ethyl methyl carbonate(EMC) was 30:20:50. Then, a negative electrode and a positive electrodewere wound with a separator interposed to form a power generatingelement, and the power generating element was housed in a prismaticbattery case made of aluminum. Thereafter, a negative electrode wasconnected to a negative electrode terminal with a negative electrodelead interposed, a positive electrode was connected to a battery lidwith a positive electrode lead interposed, and further the battery lidwas attached to the battery case by laser welding. Thereafter, anonaqueous electrolyte was injected under a reduced pressure, and anelectrolyte solution filling hole was sealed by laser welding. Thereby,a prismatic nonaqueous electrolyte secondary battery having a nominalcapacity of 400 mAh (this is referred to as a battery A) was prepared.

Example 2

A battery B was prepared in the same manner as in Example 1 except forusing amorphous carbon having an average particle size of 7.0 μm as anegative active material in the battery A of Example 1.

Comparative Example 1

A battery C was prepared in the same manner as in Example 1 except forusing amorphous carbon having an average particle size of 11.5 μm as anegative active material in the battery A of Example 1.

Comparative Example 2

A battery D was prepared in the same manner as in Example 1 except forusing amorphous carbon having an average particle size of 14.5 μm as anegative active material in the battery A of Example 1.

Comparative Example 3

A battery E was prepared in the same manner as in Example 1 except forusing amorphous carbon having an average particle size of 16.8 μm as anegative active material in the battery A of Example 1.

Example 3

A negative electrode of a battery of Example 3 was prepared in the samemanner as in Example 1 except that in the negative electrode of thebattery A of Example 1, amorphous carbon in which an average particlesize was 2.3 μm and an interlayer distance d₀₀₂, determined by a wideangle X-ray diffraction method, was 3.70 Å, was used as a negativeactive material, and the amounts of the amorphous carbon, thestyrene-butadiene rubber (SBR) as a binder and the carboxymethylcellulose (CMC) as a thickner were changed to 97 parts by mass, 2 partsby mass and 1 part by mass, respectively.

A positive electrode of a battery of Example 3 was prepared in the samemanner as in Example 1 except that in the positive electrode of thebattery A of Example 1, 88 parts by mass ofLiNi_(0.33)Co_(0.33)Mn_(0.33)O₂ was used as a positive active material,6 parts by mass of acetylene black as a conduction aid and 6 parts bymass of polyvinylidene fluoride (PVdF) were used.

A nonaqueous electrolyte of a battery of Example 3 was prepared in thesame manner as in Example 1 except that in the nonaqueous electrolyte ofthe battery A of Example 1, a nonaqueous solvent is a mixed solvent inwhich a volume ratio of ethylene carbonate (EC)/dimethyl carbonate(DMC)/ethyl methyl carbonate (EMC) was 30:20:50, and LiPF₆ was dissolvedin the nonaqueous solvent as a supporting salt so as to have theconcentration of 1 mol/l.

A battery F was prepared in the same manner as in Example 1 except thatin the battery A of Example 1, the negative electrode, the positiveelectrode and the nonaqueous electrolyte were configured as describedabove, and a nominal capacity was changed to 5.0 Ah.

Example 41

A battery G was prepared in the same manner as in Example 3 except forusing amorphous carbon having an average particle size of 3.1 μm as anegative active material in the battery F of Example 3.

Example 5

A battery H was prepared in the same manner as in Example 3 except forusing amorphous carbon having an average particle size of 4.2 μm as anegative active material in the battery F of Example 3.

Comparative Example 4

A battery I was prepared in the same manner as in Example 3 except forusing amorphous carbon having an average particle size of 9.8 μm as anegative active material in the battery F of Example 3.

[Evaluation Test] 1. Examples 1 to 2 and Comparative Examples 1 to 3(Batteries A to E) (1-1) Verification Test of Initial Capacity

In each of the batteries A to E of Examples 1 to 2 and ComparativeExamples 1 to 3, the verification test of an initial capacity wasperformed in the following charge-discharge conditions. The battery wascharged at a constant current of 400 mA at 25° C. to 3.55 V, and furthercharged at a constant voltage of 3.55 V to perform charge for 3 hours inall including constant current charge and constant voltage charge. Aftercharging, the battery was discharged at a constant current of 400 mA toan end-of-discharge voltage of 2.00 V, and this discharge capacity wasdefined as an “initial capacity”.

(1-2) Calculation of Capacity Retention Ratio (after 500 Cycle Test)

On each of the batteries A to E after the verification test of aninitial capacity, a cycle life test was performed in the followingconditions. A series of operations in which a battery was charged at aconstant current of 400 mA at 45° C. to 3.55 V, further charged at aconstant voltage of 3.55 V to perform charge for 3 hours in allincluding constant current charge and constant voltage charge, and thendischarged at a constant current of 400 mA to 2.00 V, was taken as 1cycle, and this cycle was repeated 500 times.

Then, on each of the batteries A to E after 500 cycles, a dischargecapacity was measured in the same conditions as in the verification testof an initial capacity, and a capacity retention ratio was calculated bydividing the discharge capacity by the initial capacity.

(1-3) Calculation of Relative Value of Direct-Current Resistance (Rx)

An SOC (state of charge) of a battery was set to 50% by charging each ofthe batteries A to E after the verification test of an initial capacityat a constant current of 400 mA at 25° C. to 3.20 V, and furthercharging at a constant voltage of 3.20 V for 3 hours in all, and thebattery was held at 0° C. for 5 hours. Thereafter, a voltage (E1) at thetime of discharging the battery at 80 mA (I1) for 10 seconds, a voltage(E2) at the time of discharging the battery at 200 mA (I2) for 10seconds and a voltage (E3) at the time of discharging the battery at 400mA (I3) for 10 seconds were measured. Herein, “SOC is 50%” representsthat an amount of charge is 50% with respect to the capacity of abattery.

A direct-current resistance (Rx) was calculated using theabove-mentioned measurements (E1, E2, E3). Specifically, themeasurements (E1, E2, E3) were plotted on a graph in which a horizontalaxis was a current and a vertical axis was a voltage, these threemeasurement points were approximated by a regression line (approximationline) based on a least square method, and a slope of the line was takenas a direct-current resistance (Rx).

Direct-current resistances (Rx) of the batteries A to E (Examples 1, 2and Comparative Examples 1 to 3) were relatively compared with oneanother based on the direct-current resistance (Rx) obtained in thebattery E (Comparative Example 3). That is, a relative value of thedirect-current resistance (Rx) of each of the batteries A to E to thedirect-current resistance (Rx) of the battery E was calculated from thefollowing formula (1). The direct-current resistance (Rx) of the batteryE was 816.4 mΩ.

Relative value of direct-current resistance (Rx) of each of batteries Ato E=[Direct-current resistance (Rx) of each of batteries A toE/Direct-current resistance (Rx) of battery E]×100  (1)

The capacity retention ratios (after 500 cycle test) and the relativevalues to the direct-current resistance (Rx) of the battery E of thebatteries A to E thus calculated are shown in Table 1.

TABLE 1 Average Capacity Particle Retention Size of Ratio/% RelativeValue of Amorphous (after Direct-Current Battery Carbon/μm 500 Cycles)Resistance (Rx)/% Example 1 A 5.5 86.2 73.0 Example 2 B 7.0 85.2 80.0Comparative C 11.5 67.1 104.9 Example 1 Comparative D 14.5 60.3 114.4Example 2 Comparative E 16.8 77.5 100.0 Example 3

2. Examples 3 to 5 and Comparative Example 4 (Batteries F to I) (2-1)Verification Test of Initial Capacity

In each of the batteries F to I of Examples 3 to 5 and ComparativeExample 4, the verification test of an initial capacity was performed inthe following charge-discharge conditions. The battery was charged at aconstant current of 5.0 A at 25° C. to 4.20 V, and further charged at aconstant voltage of 4.20 V to perform charge for 3 hours in allincluding constant current charge and constant voltage charge. Aftercharging, the battery was discharged at a constant current of 5.0 A toan end-of-discharge voltage of 2.50 V, and this discharge capacity wasdefined as an “initial capacity”.

(2-2) Calculation of Capacity Retention Ratio (after being Left Standingin a High-Temperature Environment)

On each of the batteries F to I after the verification test of aninitial capacity, an SOC of a battery was adjusted to 90% by chargingthe battery by 90% of the initial capacity, and then the battery wasstored for 60 days in an environment of 65° C. On each of the batteriesF to I after storing for 60 days, a discharge capacity was measured inthe same conditions as in the measurement of an initial capacity, and acapacity retention ratio was calculated by dividing the dischargecapacity by the initial capacity.

(2-3) Calculation of Relative Value of Direct-Current Resistance (Ry)

On each of the batteries F to I after the verification test of aninitial capacity, an SOC of a battery was adjusted to 50% by chargingthe battery by 50% of the initial capacity, and the battery was held at−10° C. for 4 hours. Thereafter, a voltage (E4) at the time ofdischarging the battery at 1.0 A (I4) for 10 seconds, a voltage (E5) atthe time of discharging the battery at 2.5 A (I5) for 10 seconds and avoltage (E6) at the time of discharging the battery at 5.0 A (E6) for 10seconds were measured. A direct-current resistance (Ry) was calculatedusing these measurements (E4, E5, E6). Specifically, the measurements(E4, E5, E6) were plotted on a graph in which a horizontal axis was acurrent and a vertical axis was a voltage, these three measurementpoints were approximated by a regression line (approximation line) basedon a least square method, and a slope of the line was taken as adirect-current resistance (Ry).

Direct-current resistances (Ry) of the batteries F to I (Examples 3 to 5and Comparative Example 4) were relatively compared with one anotherbased on the direct-current resistance (Ry) obtained in the battery I(Comparative Example 4). That is, a relative value of the direct-currentresistance (Ry) of each of the batteries F to I to the direct-currentresistance (Ry) of the battery I was calculated from the followingformula (2).

Relative value of direct-current resistance (Ry) of each of batteries Fto I=[Direct-current resistance (Ry) of each of batteries F toI/Direct-current resistance (Ry) of battery I]×100  (2)

The capacity retention ratios (after being left standing in ahigh-temperature environment) and the relative values to thedirect-current resistance (Ry) of the battery I of the batteries F to Ithus calculated are shown in Table 2.

TABLE 2 Capacity Retention Relative Average Ratio/% (after Value ofParticle being left Direct- Size of standing under a Current Amorphoushigh-temperature Resistance Battery Carbon/μm environment) (Ry)/%Example 3 F 2.3 82.7 64.8 Example 4 G 3.1 86.5 73.8 Example 5 H 4.2 85.883.4 Comparative I 9.8 79.8 100.0 Example 4

[Consideration]

The following matters became apparent from the results shown in Table 1.

In the battery A (Example 1) and the battery B (Example 2) in which theaverage particle sizes of the amorphous carbon as the negative activematerial were 7 μm or less, relative values of the direct-currentresistances (Rx) to the direct-current resistance (Rx) of the battery Ewere 80% or less, and the capacity retention ratios (after 500 cycles)were 85% or more. In the batteries C to E (Comparative Examples 1 to 3)in which the average particle sizes of the amorphous carbon as thenegative active material were larger than 7 μm, relative values of thedirect-current resistances (Rx) to the direct-current resistance (Rx) ofthe battery E were 100% or more, and the capacity retention ratios(after 500 cycles) were 80% or less. In the batteries A to E (Examples1, 2 and Comparative Examples 1 to 3), it was found that relative valuesof the direct-current resistances (Rx) of the batteries A and B(Examples 1 and 2), in which the average particle size of the amorphouscarbon is small, to the direct-current resistance (Rx) of the battery E,are smaller than those of the batteries C to E (Comparative Examples 1to 3), and power characteristics tend to increase. Further, in thebatteries A to E (Examples 1, 2 and Comparative Examples 1 to 3), whenthe average particle size of the amorphous carbon is reduced, thecapacity retention ratio turned from decrease to increase at the averageparticle size (14.5 μm) of the amorphous carbon corresponding to thebattery D (Comparative Example 2) as a boundary. The reason for this issupposed that the average particle size of the amorphous carbon as aboundary at which the capacity retention ratio turns from decrease toincrease with a reduction of the average particle size of the amorphouscarbon, exists between the average particle size (16.8 μm) of theamorphous carbon corresponding to the battery E (Comparative Example 3)and the average particle size (11.5 μm) of the amorphous carboncorresponding to the battery C (Comparative Example 1).

Although a factor that the capacity retention ratio turns from decreaseto increase with a reduction of the average particle size of theamorphous carbon is not clear, it is thought as the factor that theaqueous binder strongly interacts with the surface of the amorphouscarbon particle. Since the amorphous carbon is fired and produced at atemperature lower than other carbon materials, it is thought that alarge amount of a surface functional group (including hydrophilic groupssuch as a hydroxy group (—OH) and an oxo group (═O)) remains, and theaqueous binder strongly interacts with the surface of the amorphouscarbon resulting from the surface functional group. That is, it isthought that the aqueous binder more strongly interacts with the surfaceof the amorphous carbon since the amount of the surface functional groupis increased by reducing the average particle size of the amorphouscarbon to 7 μm or less. Thereby, the activity of the particle surface ofthe amorphous carbon is lowered to suppress a decomposition reaction ofthe nonaqueous electrolyte at the particle surface of the amorphouscarbon to increase the capacity retention ratio.

Further, when the cellulose-based polymers (e.g., alkyl cellulose andsalts thereof) are used as the thickner contained in the negativecomposite layer, the thickner is thought to interact with the surfacesof the amorphous carbon particles since the thickner includessubstitutes such as a hydroxy group and a carboxymethyl group. That is,since the negative composite layer includes the thickner, it is thoughtthat the activity of the surface of the amorphous carbon particle isfurther lowered.

The cellulose-based polymers are not particularly limited; and theypreferably include carboxymethyl cellulose (CMC). Further, a degree ofetherification of the cellulose-based polymer is not particularlylimited; however, it is preferably 1 or less since it is thought thatthe hydroxyl groups exist in large numbers to further lower the activityof the surface of the amorphous carbon particle.

The present inventors made investigations concerning a battery includinga negative electrode using an aqueous binder, and the present inventorsfound that the capacity retention ratio of a battery is improvedcontrary to conventional technical common knowledge. The improvement ismade by setting the average particle size of the amorphous carbon as thenegative active material to a value smaller than the specific particlesize presents between 11.5 μm and 16.8 μm. This cannot be easilyconceived by even those skilled in the art.

Further, that the capacity retention ratio is improved by setting theaverage particle size of the amorphous carbon as the negative activematerial to a value smaller than the specific particle size between 11.5μm and 16.8 μm is supposed to be an effect achieved based on containingthe aqueous binder in the negative electrode.

The following matters became apparent from the results shown in Table 2.

In the batteries F to H (Examples 3 to 5) in which the negativeelectrode included amorphous carbon as a negative active material and anaqueous binder, and the average particle sizes of the amorphous carbonparticles were 7 μm or less, specifically, the average particle sizeswere set to 2.3 μm, 3.1 μm, and 4.2 μm, respectively, relative values ofthe direct-current resistances (Ry) to the direct-current resistance(Ry) of the battery I were 85% or less, and the capacity retentionratios (after being left standing in a high-temperature environment)were 80% or more. In the battery I (Comparative Example 4) in which theaverage particle size of the amorphous carbon as the negative activematerial was larger than 7 μm, the relative value of the direct-currentresistance (Ry) to the direct-current resistance (Ry) of the battery Iwas 100%, and the capacity retention ratio (after being left standing ina high-temperature environment) was less than 80%. The reason why thebatteries F to H (Examples 3 to 5) exhibit high capacity retention ratioand high power characteristics as with the batteries A and B (Examples 1and 2) is supposed that as with above description, the negativeelectrode included amorphous carbon as a negative active material and anaqueous binder, and the average particle sizes of the amorphous carbonparticles were set to 7 μm or less.

From these results, it was found that when the negative electrodeincludes amorphous carbon as a negative active material and an aqueousbinder, and the average particle sizes of the amorphous carbon particlesare set to 7 μm or less, the power characteristics and the capacityretention ratio can be improved.

The embodiments disclosed in the present specification and the exampleswhich are implementation thereof are intended to illustrate theinvention in all respects and are not to be construed to limit theinvention. It will be readily understood that those skilled in the artcan appropriately modify the above-mentioned embodiments and exampleswithout departing from the gist of the invention. Accordingly,naturally, another embodiment modified without departing from the gistof the invention is embraced by the scope of the invention.

For example, the positive electrode material, the nonaqueous electrolyteand the like can be appropriately selected in accordance withperformance/specification required of the nonaqueous electrolytesecondary battery.

Further, for example, as the aqueous binder contained in the negativeelectrode, various compounds having specified characteristics can beemployed without being limited to the compounds exemplified in thepresent specification.

Further, for example, with respect to a shape of the nonaqueouselectrolyte secondary battery, a cylindrical or a laminate-shapednonaqueous electrolyte secondary battery can be used without beinglimited to a prismatic shape.

The present invention can realize an energy storage apparatus using anassembled battery formed by combining a plurality of the nonaqueouselectrolyte secondary batteries of the present invention, and oneembodiment thereof is shown in FIG. 2. The energy storage apparatusincludes a plurality of energy storage units 20. Each energy storageunit 20 is composed of the assembled battery including a plurality ofthe nonaqueous electrolyte secondary batteries 1. An energy storageapparatus 30 can be installed as a power supply for automobiles such aselectric automobiles (EV), hybrid automobiles (HEV) and plug-in hybridautomobiles (PHEV).

The energy storage apparatus 30 in which the nonaqueous electrolytesecondary battery of the present invention is used can be installed onan automobile 100 as a power supply for automobiles such as electricautomobiles (EV), hybrid automobiles (HEV) and plug-in hybridautomobiles (PHEV), and one embodiment thereof is shown in FIG. 3.Further, since the nonaqueous electrolyte secondary battery of thepresent invention has high power characteristic, it is preferably usedfor an automobile power supply of hybrid automobiles (HEV) or anautomobile power supply of plug-in hybrid automobiles (PHEV), and morepreferably used for the automobile power supply of hybrid automobiles(HEV).

Further, for example, with respect to a subject which plays a role ofelectric conduction, cations of alkali metals such as sodium, potassiumand cesium; cations of alkaline-earth metals such as calcium and barium;and cations of other metals such as magnesium, aluminum, silver andzinc, can be used without limiting to lithium ions. That is, anotheralkali metal ion secondary battery may be used.

INDUSTRIAL APPLICABILITY

The present invention can be used for nonaqueous electrolyte secondarybatteries such as a lithium ion secondary battery. Since the nonaqueouselectrolyte secondary battery according to the present invention hasexcellent power characteristics and an excellent capacity retentionratio, it can be effectively used for a power supply for automobilessuch as electric automobiles (EV), hybrid automobiles (HEV) and plug-inhybrid automobiles (PHEV), a power supply for electronic equipment, anda power supply for electric power storage.

DESCRIPTION OF REFERENCE SIGNS

-   -   1 Nonaqueous electrolyte secondary battery    -   2 Power generating element    -   3 Positive electrode (positive electrode plate)    -   4 Negative electrode (negative electrode plate)    -   5 Separator    -   6 Battery case    -   7 Battery lid    -   8 Safety valve    -   9 Negative electrode terminal    -   10 Positive electrode lead    -   11 Negative electrode lead    -   20 Energy storage unit    -   30 Energy storage apparatus    -   40 Automobile main body    -   100 Automobile

1. A nonaqueous electrolyte secondary battery comprising: a negativeelectrode including amorphous carbon as a negative active material and abinder, wherein the binder contains an aqueous binder, and wherein anaverage particle size of the amorphous carbon is 7 μm or less.
 2. Thenonaqueous electrolyte secondary battery according to claim 1, whereinthe aqueous binder contains at least one selected from among rubber-likepolymers and resin-based polymers which can be dissolved or dispersed ina water-based solvent.
 3. The nonaqueous electrolyte secondary batteryaccording to claim 1, wherein an interlayer distance d₀₀₂, which isdetermined by a wide angle X-ray diffraction method, of the amorphouscarbon is 3.60 Å or more.
 4. The nonaqueous electrolyte secondarybattery according to claim 1, wherein the negative electrode includes athickner, and wherein the thickner contains a cellulose-based polymer.5. The nonaqueous electrolyte secondary battery according to claim 4,wherein the cellulose-based polymer includes a carboxymethyl cellulose.6. The nonaqueous electrolyte secondary battery according to claim 4,wherein a degree of etherification of the cellulose-based polymer is 1or less.
 7. An assembled battery including a plurality of the nonaqueouselectrolyte secondary batteries according to claim
 1. 8. An energystorage apparatus including the assembled battery according to claim 7.9. An automobile equipped with the energy storage apparatus according toclaim
 8. 10. A nonaqueous electrolyte secondary battery comprising: anegative electrode including amorphous carbon as a negative activematerial and a binder, wherein the binder contains an aqueous binder,and wherein an average particle size of the amorphous carbon is 7 μm orless, wherein the aqueous binder contains at least one selected fromamong rubber-like polymers and resin-based polymers which can bedissolved or dispersed in a water-based solvent, and wherein aninterlayer distance d₀₀₂, which is determined by a wide angle X-raydiffraction method, of the amorphous carbon is 3.60 Å or more.
 11. Thenonaqueous electrolyte secondary battery according to claim 10, whereinthe negative electrode includes a thickner, and wherein the thicknercontains a cellulose-based polymer.
 12. The nonaqueous electrolytesecondary battery according to claim 11, wherein the cellulose-basedpolymer includes a carboxymethyl cellulose.
 13. The nonaqueouselectrolyte secondary battery according to claim 11, wherein a degree ofetherification of the cellulose-based polymer is 1 or less.